1. Field of the Invention
This invention relates to magnetic materials for magnetic recording media, such as magnetic recording media for high density recording containing these magnetic materials. In particular, this invention relates to the production of video tapes using ferromagnetic metal powder materials of a novel composition.
2. Description of the Prior Art
Ferromagnetic powder materials heretofore used in video tapes include .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-containing .gamma.-Fe.sub.2 O.sub.3 and CrO.sub.2.
The recording wavelengths in video tapes are far shorter than those of other sound recording tapes, and for example, it is necessary to record short waves of at least up to a minimum wavelength of 6 .mu. or so in a VTR (video tape recorder) for television broadcast. The above-described conventional ferromagnetic materials are not as suitable for magnetic recording of recording signals of a short wavelength (of about 10 .mu. or less). That is, these ferromagnetic materials have insufficient magnetic characteristics of coercive force (Hc) and residual magnetic flux density for use in high density recording. Miniaturization and simplification of recording and reproducing mechanisms has occurred recently, and development of a small size VTR having a stationary head has been accelerated in this technical field in substitution for conventional rotary head type VTR's. An especially important element in a stationary head VTR is how to reduce the relative velocity between the tape and the head.
More precisely, the minimum recording wavelength is desirably at most 2 .mu. or less so as to avoid the high speed conveyance of a video tape in a small size VTR. Recently, extensive development of ferromagnetic powder materials having magnetic characteristics suitable for high density recording has been carried out. One of the subject materials is a ferromagnetic metal powder. The powder is made of a metal or a metal alloy. This powder additionally includes an intermetallic compound. Metals which are mainly used include iron, cobalt and nickel, and other elements such as chromium, manganese, rare earth elements and zinc are optionally added thereto. For preparation of these ferromagnetic materials the following methods are known:
I. A salt of a ferromagnetic metal and an organic acid is pyrolyzed and reduced with a reductive gas, for example, as disclosed in Japanese Pat. Nos. 11412/66 and 38417/72.
II. A needle-shaped oxyhydroxide, or a derivative thereof containing other metals, or a needle-shaped iron oxide obtained from these oxyhydroxide substances is reduced, for example, as disclosed in Japanese Pat. Nos. 3862/60 and 39477/72 and British Pat. No. 1,192,167.
III. A ferromagnetic metal is evaporated in an inert gas, for example, as disclosed in Japanese Pat. No. 27718/72 and Ohyo Butsuri (Applied Physics), Vol. 40, No. 1, page 110 (1970).
IV. A metal carbonyl compound is decomposed, for example, as disclosed in U.S. Pat. Nos. 2,983,997 and 3,228,882.
V. A ferromagnetic metal is deposited by electrodeposition using a mercury electrode, and then separated from the mercury, for example, as disclosed in Japanese Pat. Nos. 15525/64 and 8123/65.
VI. A salt of a ferromagnetic metal material is reduced in a solution thereof, for example, as disclosed in Japanese Pat. Nos. 20520/63, 26555/63, 20116/68 and 41718/72, U.S. Pat. Nos. 3,663,318 and 3,661,556 and German patent application laid-open to public inspection, OLS No. 2,132,430.
Investigations have revealed that a magnetic recording medium having a ferromagnetic material prepared by the above Method (VI) is relatively free from noise and has better surface chaeracteristics, when used as a video tape, as compared with other tapes having other ferromagnetic materials prepared by other methods.
Accordingly, the ferromagnetic materials used in the present invention are those prepared by Method (VI), and in particular, a borohydride compound or a derivative thereof such as borane, borazane, borohydride, sodium borohydride, potassium borohydride, dimethylaminoborane or diethylaminoborane is used as the reducing agent in Method (VI).
Method (VI) wherein a salt of a ferromagnetic metal material is reduced in a solution thereof with borohydride compound or a derivative thereof has some defects as described below. The powders obtained according to this method are less resistant to moisture, particularly when iron is present, and are gradually oxidized even at normal temperature, in an extreme case, resulting in a loss of the magnetic characteristics thereof to become consequently a non-magnetic substance. According to this method, furthermore, since the surface activity of the particles obtained is high, the particles are highly active. This high activity is industrially disadvantageous from the standpoint of the production thereof. Use of highly active powders in air is dangerous due to their explosive character. In addition, these powders, when kneaded with a binder, tend to deteriorate the binder used, often resulting in a disadvantage in dispersion and coating. A tape utilizing a magnetic substance obtained in Method (VI) is defective in that the rubbing of the tape against a video-head at a high relative speed often deteriorates the magnetic material in the tape (presumably the material is oxidized). All of these defects must necessarily be overcome in the development of magnetic recording media for high density recording. However, it has been extremely difficult up to the present to solve all of these problems. For example, it is possible to manufacture magnetic materials capable of having a high Hc and a high squareness ratio, however, it is difficult to manufacture in Method (VI) magnetic materials of a low Hc and a high squareness ratio.